You've muddled the definitions again. "opt" signifies an action by the user.
If I am "in a group" by default, then I can take an action to "opt out", requesting to be removed from the list.
If I am not initially joined to the group, then I can take an action to "opt in" and be added to the list.
There is no such thing as "opt by default". That is not a user action. It also makes no sense for the same list or group to be both "opt in" and "opt out" because, as adjectives, they imply the default states and they describe the user action taken to change that default.
https://en.wiktionary.org/wiki/opt-in
You've muddled the definitions again. "opt" signifies an action by the user.
If I am "in a group" by default, then I can take an action to "opt out", requesting to be removed from the list.
If I am not initially joined to the group, then I can take an action to "opt in" and be added to the list.
There is no such thing as "opt by default". That is not a user action. It also makes no sense for the same list or group to be both "opt in" and "opt out" because, as adjectives, they imply the default states and they describe the user action taken to change that default.
opt-in: default state is out
opt-out: default state is in